Comprehensive and Detailed in Depth Explanation:
To connect to an HCP Vault cluster using the Vault CLI, you need to setVAULT_ADDRand VAULT_NAMESPACE. The HashiCorp Vault documentation states: "You can use environment variables to configure the CLI globally. For example, export VAULT_ADDR='http://localhost:8200' sets the address of your Vault server globally." For HCP Vault, the default port is 8200, and the default namespace is "admin," so VAULT_ADDR=https://<cluster-address>:8200 and VAULT_NAMESPACE=admin are required. A token (via VAULT_TOKEN) is also needed for authentication but is typically set after initial connectivity.
VAULT_CLIENT_KEYisn't a standard variable for CLI connectivity.VAULT_REDIRECT_ADDRand VAULT_CLUSTER_ADDRare not used for this purpose. Thus, C provides the correct variables.
Reference:
HashiCorp Vault Documentation - CLI Environment Variables

Comprehensive and Detailed in Depth Explanation:
For an application that cannot manage authentication with Vault but can communicate with a local service, the Vault Proxy with Auto-Auth feature enabledis the optimal solution. The HashiCorp Vault documentation states that Vault Proxy can "act as a proxy between Vault and the application, optionally simplifying the authentication process." The Auto-Auth feature allows the proxy to handle authentication on behalf of the application, enabling it to generate dynamic credentials without the application needing to manage the authentication process directly. This aligns perfectly with the requirement of delegating authentication to a local service.
Vault Proxy with cachingimproves performance by caching responses but does not inherently handle authentication, missing the core need.Vault Agent with environment variable secret injectioninjects secrets into the application's environment but assumes the agent manages authentication, which the application cannot do.Vault Agent with templatinggenerates credentials based on templates but still requires authentication management, which the application cannot handle. Vault Proxy with Auto-Auth uniquely addresses this by offloading authentication responsibilities.
Reference:
HashiCorp Vault Documentation - Vault Agent and Proxy

Vault policies are written in HCL or JSON format and are attached to tokens or roles by name. Policies define the permissions and restrictions for accessing and performing operations on certain paths and secrets in Vault. Policies are deny by default, which means that an empty policy grants no permission in the system, and any request that is not explicitly allowed by a policy is implicitly denied1. Some of the features and benefits of Vault policies are:
* Policies are path-based, which means that they match the request path to a set of rules that specify the allowed or denied capabilities, such as create, read, update, delete, list, sudo, etc2.
* Policies are additive, which means that if a token or a role has multiple policies attached, the effective policy is the union of all the individual policies. The most permissive capability is granted if there is a conflict3.
* Policies can use glob patterns, such as * and +, to match multiple paths or segments with a single rule. For example, path "secret/*" matches any path starting with secret/, and path "secret/+/config" matches any path with two segments after secret/ and ending with config4.
* Policies can use templating to interpolate certain values into the rules, such as identity information, time, randomness, etc. For example, path "secret/{{identity.entity.id}}/*" matches any path starting with secret/ followed by the entity ID of the requester5.
* Policies can be managed by using the vault policy commands or the sys/policy API endpoints. You can write, read, list, and delete policies by using these interfaces6.
The default policy is a built-in policy that is attached to all tokens by default and cannot be deleted. However, the default policy can be modified by using the vault policy write command or the sys/policy API endpoint. The default policy provides common permissions for tokens, such as renewing themselves, looking up their own information, creating and managing response-wrapping tokens, etc7.
You do not have to use YAML to define policies, as Vault supports both HCL and JSON formats. HCL is a human-friendly configuration language that is also JSON compatible, which means that JSON can be used as a valid input for policies as well8.
Vault does not need to be restarted in order for a policy change to take effect, as policies are stored and evaluated in memory. Any change to a policy is immediately reflected in the system, and any token or role that has that policy attached will be affected by the change.: 1(https://developer.hashicorp.com/vault/docs/concepts/policies), 2(https://developer.hashicorp.com/vault
/docs/concepts/policies), 3(https://developer.hashicorp.com/vault/docs/concepts/policies), 4(https://developer.
hashicorp.com/vault/docs/concepts/policies), 5(https://developer.hashicorp.com/vault/docs/concepts
/policies), 6(https://developer.hashicorp.com/vault/docs/commands/lease), 7(https://developer.hashicorp.com
/vault/docs/concepts/policies), 8(https://developer.hashicorp.com/vault/docs/concepts/policies), (https://developer.hashicorp.com/vault/docs/concepts/policies#policy-updates)

Comprehensive and Detailed in Depth Explanation:
For a legacy application that cannot communicate directly with Vault but needs secrets in a local configuration file, theVault Agent with templating featureis the best solution. The HashiCorp Vault documentation notes: "Vault Agent can obtain secrets and provide them to applications," and its templating feature "generates dynamic credentials based on predefined templates" and writes them to files. This allows secrets to be automatically fetched and rendered into a configuration file, meeting the requirement without refactoring.
Vault Proxy with Auto-Authsimplifies authentication but doesn't inherently write secrets to files.Vault Proxy as an API proxysecures API access but doesn't address file writing.Vault Agent with caching improves performance but doesn't solve the file output need. Thus, A is correct.
Reference:
HashiCorp Vault Documentation - Vault Agent and Proxy

When unsealing Vault, each Shamir unseal key should be entered by different administrators each connecting from different computers. This is because the Shamir unseal keys are split into shares that are distributed to trusted operators, and no single operator should have access to more than one share. This way, the unseal process requires the cooperation of a quorum of key holders, and enhances the security and availability of Vault. The unseal keys can be entered via multiple mechanisms from multiple client machines, and the process is stateful. The order of the keys does not matter, as long as the threshold number of keys is reached.
The unseal keys should not be entered at the command line in one single command, as this would expose them to the history and compromise the security. The unseal keys should not be encrypted with each administrator's PGP key, as this would prevent Vault from decrypting them and reconstructing the master key. References: https://developer.hashicorp.com/vault/docs/concepts/seal3, https://developer.hashicorp.com
/vault/docs/commands/operator/unseal